Why is my Geometry Shader meshes moving when I move my camera?

Hello, I have a basic geometry shader that I use to generate quads and texture them. Every time I move my camera, the quads also seem to rotate in the direction of my camera (they are always perpendicular to my camera), I don't know why that is happening.

If I remove Projection * View then the grass wouldn't be visible to the camera anymore.

It's very possible, because the displacement would be happening in object-space, which may have any orientation w.r.t. the camera (eye-space and clip-space). It could even be edge-on, so nothing would be rendered (i.e. no pixels would be rasterized).

This is why billboards are typically positioned orthogonal to the camera in screen-space or eye-space. Their purpose not to define the boundary of a physical object, but rather to just trigger rasterization and shading for these pixels so you can render something that looks reasonable for an object that's not so easily defined (e.g. fuzzy smoke cloud, blades of grass, fire, cumulus cloud, font character, etc.)